The present invention concerns a video signal encoder and decoder system. More particularly, the present invention relates to apparatus and methods for encoding and decoding a television signal so as to prevent others from having unauthorized access to the signal.
In present-day pay television systems, it is often necessary to encode or "scramble" a transmitted video signal to control access to the signal by the consuming public. A variety of types of encoding are employed, all utilizing the concept of rendering the video signal unviewable to a normal television receiver without having proper decoding apparatus. In subscription television systems, an encoder/decoder system is essential to prevent unauthorized reception of the signals which are transmitted by radio frequency. Even in systems which are generally "closed" to the public, for example, coaxial cable systems, it has become increasingly important to scramble the signals at the transmitter or "headend" of the system. Although access to the video signals on cable systems is substantially limited by the very nature of the transmission via coaxial cable, it is necessary to control the continuation of services and access to certain levels of programming for subscribers as part of an on-going cable service.
It is not uncommon for a typical cable television system to provide signals on fifty-five or more channels, in contrast to a subscription television system which may transmit only one or two television channels. Cable systems, therefore, require encoder/decoder systems having apparatus capable of processing many channels without being inordinately expensive. Encoder/decoder apparatus for cable systems should utilize scrambling and descrambling components which do not require a high degree of accuracy or have precise timing requirements which would substantially increase the cost of the components. Moreover, it is preferable to minimize or eliminate any additional transmission of control signals or other additional data which would be required to decode the scrambled signal.
In some encoding systems, an approach is used of randomly scrambling the video data information and/or the synchronizing pulses of the video signal. Such a system invariably requires the transmission of additional control data or signals to provide the necessary information for a decoder unit to unscramble the video signal at the receiver. The transmission of such additional data is both inefficient and requires detection components in the decoder to change the video signal back to its proper format.
Other scrambling systems suppress certain signals, particularly the horizontal synchronization pulses of the video signal so that they cannot be recognized by a television receiver without proper decoding equipment. However, some television receivers today are sufficiently sophisticated to be able to lock onto a video signal even with the horizontal synchronization pulses suppressed to at least provide a black and white picture for the viewer. In some cases, the receivers can even detect some color from such scrambled signals although the color might be altered.
Thus, it has become important, particularly for a cable television system to have encoding and decoding units having relatively inexpensive and simple components requiring little if any transmission of control signals to decode the video signal. It is also desirable to have a scrambling system which thoroughly encodes the signal so that it cannot be even partially decoded without authorized circuitry.